Semiconductor light emitting devices (“LEDs”), such as light emitting diodes and laser diodes are widely known solid-state lighting elements that are capable of generating light upon application of voltage thereto. Light emitting devices generally include a p-n junction, an anode ohmic contact for the p-type region of the device, and a cathode ohmic contact for the n-type region of the device. The device may be formed on a substrate, such as a sapphire, silicon, silicon carbide, gallium arsenide, gallium nitride, etc., substrate, or the device may not include a substrate.
LEDs are increasingly being used in lighting/illumination applications, with an ultimate goal being a replacement for the ubiquitous incandescent light bulb. As is well known to those having skill in the art, an incandescent light bulb generally produces “warm white light”, i.e., light with a Correlated Color Temperature (CCT) of between about 2,600 K and about 6,000 K. In order to produce white light, multiple LEDs emitting light of different colors and/or one or more single color LEDs that include a wavelength conversion material such as phosphor thereon, may be used.
In efforts to increase the total light output of an LED, researchers and manufacturers have continued to apply increasing current density to the LED. Unfortunately, a major obstacle to increasing the current density in the LED is so-called “droop”, i.e., the falloff in quantum efficiency at relatively high drive currents.
FIG. 1 illustrates the droop phenomena by graphically illustrating current density vs. quantum efficiency (for example in lumens per watt or as a %) for a conventional LED. As shown in FIG. 1, at relatively low current densities, a linear increase in quantum efficiency is attained. However, at increasing current densities, a decrease in quantum efficiency from the linear quantum efficiency shown by the dashed line of FIG. 1 takes place, as shown by the right pointing arrow in FIG. 1. The decrease in quantum efficiency indicated by the cross-hatched area between the solid line of actual quantum efficiency and the dashed line of the linear quantum efficiency, is referred to as “droop”. Notwithstanding this droop, researchers and manufacturers of LEDs have continued to develop LEDs that can sustain increased current density in an attempt to maximize the light output of the device.